Protobuf and FIDL have very different design goals though. One is meant for backwards compatible over the wire communication between loosely coupled RPC systems, and one is meant to be a highly efficient IPC.

I expect, for example, that it is vastly more efficient to send a known datastructure over the wire in FIDL, but it is vastly more efficient to parse, modify/extend a repeated subfield, and then reserialize a protobuf.

You can't "fix" a system to achieve two goals that are in conflict, sometimes it is in fact better to have N opinionated systems, than one that is only okay at everything.